Recently, a nonaqueous electrolyte battery such as a lithium ion secondary battery has been actively researched and developed as a high energy-density battery. The nonaqueous electrolyte battery is expected to be used as a power source for hybrid vehicles, electric cars, an uninterruptible power supply for base stations for portable telephone, or the like. Therefore, the nonaqueous electrolyte battery is demanded to, in addition to having a high energy density, be excellent in other performances such as rapid charge-and-discharge performances and long-term reliability, as well. For example, a nonaqueous electrolyte battery enabling rapid charge-and-discharge not only remarkably shortens a charging time but also makes it possible to improve performances related to motivity and to efficiently recover regenerative energy from motivity, in a hybrid vehicle or the like.
In order to enable rapid charge-and-discharge, electrons and lithium ions must be able to migrate rapidly between the positive electrode and the negative electrode. However, when a battery using a carbon-based negative electrode is repeatedly subjected to rapid charge-and-discharge, dendrite precipitation of metallic lithium occurs on the electrode, raising concern to heat generation and fires caused by internal short circuits.
In light of this, a battery using a metal composite oxide in place of a carbonaceous material in the negative electrode has been developed. In particular, in a battery using titanium oxide as the negative electrode active material, rapid charge-and-discharge can be stably performed. Such a battery also has a longer life than those using a negative electrode with carbonaceous material.
However, compared to carbonaceous materials, titanium oxide has a higher potential relative to metallic lithium. That is, titanium oxide is more noble. Furthermore, titanium oxide has a lower capacity per weight. Therefore, a battery using titanium oxide as the negative electrode active material has a problem that the energy density is lower. In particular, when a material having a high potential relative to metallic lithium is used as a negative electrode material, the voltage becomes lower than that of a conventional battery using a carbonaceous material. Therefore, when the battery is used for systems requiring a high voltage such as an electric vehicle and a large-scale electric power storage system, there is a problem that the number of batteries connected in a series becomes large.
The potential of the electrode using titanium oxide is about 1.5 V relative to metallic lithium and is higher (more noble) than that of the negative electrode with carbonaceous material. The potential of titanium oxide arises from the redox reaction between Ti3+ and Ti4+ when lithium is electrochemically absorbed and released, and is therefore limited electrochemically. It has therefore been conventionally difficult to drop the potential of the electrode in order to improve the energy density.